Radio frequency (RF) devices suffer from parasitic losses due to coupling to the substrate. One approach to improve performance of such RF devices is to use highly resistive substrates, which are known to reduce parasitic capacitance and coupling to the substrate. The use of highly resistive substrates offers the advantages of better integration of passive devices, reduction of cross talk, as well as a reduction of power consumption through charging and discharging of unwanted capacitances.
However, to maintain stability in such highly resistive substrates, very low oxygen concentrations are required. That is, utilizing highly resistive silicon substrates requires strict specifications on oxygen concentration to prevent thermal donors from arising during back end low temperature anneals (e.g., 400° C.-600° C.). As is known, thermal donors can have unwanted/undesired effects on the integrated circuit (IC) performance such as increased depletion region (when counter-doping initial boron doping), and unintended doping. It is important to note that it is very difficult to reduce thermal donors once they arise in the back end annealing process because it requires high temperature annealing.
Wafers having highly resistive substrates, i.e., approximately 1,000 ohm-cm to 3,000 ohm-cm resistivity range, which meet strict oxygen specifications can be very expensive and difficult to control. As to the latter drawback, the various tight restrictions to doping and oxygen concentration are required to ensure that few oxygen donors are activated during the many anneal steps during back of the line processes. Also, highly resistive substrates provided directly from a manufacture may have quality control issues; that is, there is no guarantee that a manufacturer can provide the required substrate resistivity, on a consistent basis. Also, using such wafers will limit back end anneals which may affect the number of back end metal levels offered by the technology.
To solve the above problems, it is known to build integrated circuits (IC) on silicon on insulator (SOI) wafers. However, SOI wafers are also expensive and they may exhibit defect density, etc., when compared to bulk Si substrates. As an alternative approach, it is possible to transfer fully processed wafers onto alternative substrates such as glass, AlN, alumina, ferrite, PCB, etc. However, this also is an expensive process, adding to both fabrication complexity and time.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.